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Removing characters from strings faster with AVX-512

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Re: Removing characters from strings faster with AVX-512

#31

Post by R0b0t1 »

PragmaticPulp wrote: post #30
Andoryuuta wrote: post #22

Intel is removing AVX-512 support from their newer CPU's (Alder Lake +). :/ https://www.igorslab.de/en/intel-deactivated-avx-512-on-alde...

Server and workstation chips still have AVX-512. It’s only unsupported on CPUs with smaller E(fficeincy) cores. AVX-512 was never really supported in newer consumer CPUs with heterogeneous architecture. These CPUs have a mix of powerful cores and efficiency cores. The AVX-512 instructions were never added to the efficiency cores because it would use way too much die space and defeat the purpose of efficiency cores. T…

That's not a valid reason why I can't use them on the P cores. Some motherboards can enable them on the i9-12900k, it works fine, but you need to pin to a P core.

Re: Removing characters from strings faster with AVX-512

#33

Post by PragmaticPulp »

R0b0t1 wrote: post #31
PragmaticPulp wrote: post #30

Earlier quoted context omitted.

Server and workstation chips still have AVX-512. It’s only unsupported on CPUs with smaller E(fficeincy) cores. AVX-512 was never really supported in newer consumer CPUs with heterogeneous architecture. These CPUs have a mix of powerful cores and efficiency cores. The AVX-512 instructions were never added to the efficiency cores because it would use way too much die space and defeat the purpose of efficiency cores. T…

That's not a valid reason why I can't use them on the P cores. Some motherboards can enable them on the i9-12900k, it works fine, but you need to pin to a P core.

The reason is that it was never validated or tested with AVX-512 and Intel and motherboard vendors couldn’t commit to shipping everything with AVX-512 support in future steppings/revisions.

If you disable E cores you could enable AVX-512 on certain motherboards, but like I said that’s not really a net win 99.99% of the time when you’re giving up entire cores.

It was also at your own risk because presumably the power/clock speed profiles were never tuned for a feature that wasn’t actually supported. I can see exactly why they turned it off on newer CPUs only after an announcement.

Re: Removing characters from strings faster with AVX-512

#34

Post by pclmulqdq »

fulafel wrote: post #21
jquery wrote: post #18

I prefer AMDs approach that allows them to put more cores on the die instead of supporting a rarely used instruction set.

Zen 4 is rumored to have AVX512. AMD has in the past had support for wide SIMD instructions with half internal width implementation, so the die area requirements and instruction set support are somewhat orthogonal. There's many other interesting things in AVX512 besides the wide vectors.

AVX-512 finally gets a lot of things right about vector manipulation and plugged a lot of the holes in the instruction set. Part of me is upset that it came with the "512" name - they could have called it "AVX3" or "AVX Version 2" (since it's intel and they love confusing names).

Re: Removing characters from strings faster with AVX-512

#35

Post by vlovich123 »

SemanticStrengh wrote: post #27
gslin wrote: post #11

A problem is slowing down the CPU frequency significantly when AVX-512 is involved, e.g. https://en.wikichip.org/wiki/intel/xeon_gold/6262v this, which usually cancels out the benefit in the Real World (tm).

Please stop spreading this fallacy, while downclocking can happen, usually the benefit is still strong and superior to avx256. Even 256 can induce downclocking. AVX 512 when properly utilized simply demolish non AVX 512 cpus.

On that one task. The challenge is if the avx512 pieces aren’t a bottleneck in every single concurrent workload you run. It’s fine if the most important thing your running on them is code optimized for AVX512. Realistically though, is that the case for the target market of CPUs capable of AVX512, since consumer use cases aren’t? The predominant workload would be cloud right? Where you’re running heterogeneous workloads right? You’d have to get real smart by coalescing AVX512 and non AVX512 workloads onto separate machines and disabling it on the machines that don’t need it. Very complicated work to do because you’d have to have each workload annotated by hand (memcpy is optimized to use AVX512 when available so the presence of AVX512 in the code is insufficient)

The more generous interpretation is that Intel fixed that issue a while back although the CPUs with that problem are still in rotation and you have to think about that when compiling your code.

Re: Removing characters from strings faster with AVX-512

#36

Post by PragmaticPulp »

gslin wrote: post #11

A problem is slowing down the CPU frequency significantly when AVX-512 is involved, e.g. https://en.wikichip.org/wiki/intel/xeon_gold/6262v this, which usually cancels out the benefit in the Real World (tm).

This was massively exaggerated by journalists when AVX-512 was first announced.

It is true that randomly applied AVX-512 instructions can cause a slight clock speed reduction, the proper way to use libraries like this would be within specific hot code loops where the mild clock speed reduction is more than offset by the huge parallelism increase.

This doesn’t make sense if you’re a consumer doing something multitasking and a background process is invoking the AVX-512 penalty in the background, but it usually would make sense in a server scenario.

Re: Removing characters from strings faster with AVX-512

#38

Post by tialaramex »

jcranmer wrote: post #28
_rtld_global_ro wrote: post #8

Earlier quoted context omitted.

Given Intel's AVX extension could cause silent failures on servers (very high work load for prolonged time, compare to end user computers), I'm not sure it would be a big win for servers either: https://arxiv.org/pdf/2102.11245.pdf .

I'm downvoting you because the assertion you're implying--that use of AVX increases soft failure rates more than using non-AVX instructions would--is not sustained by the source you use as reference.

Indeed, I'd summarise that source as "At Facebook sometimes weird stuff happens. We postulate it's not because of all the buggy code written by Software Engineers like us, it must be hardware. As well as lots of speculation about hypothetical widespread problems that would show we're actually not writing buggy software, here's a single concrete example where it was hardware".

If anything I'd say that Core 59 is one of those exceptions that prove the rule. This is such a rare phenomenon that when it does happen you can do the work to pin it down and say yup, this CPU is busted - if it was really commonplace you'd constantly trip over these bugs and get nowhere. There probably isn't really, as that paper claims, a "systemic issue across generations" except that those generations are all running Facebook's buggy code.

Re: Removing characters from strings faster with AVX-512

#39

Post by pantalaimon »

PragmaticPulp wrote: post #30
Andoryuuta wrote: post #22

Intel is removing AVX-512 support from their newer CPU's (Alder Lake +). :/ https://www.igorslab.de/en/intel-deactivated-avx-512-on-alde...

Server and workstation chips still have AVX-512. It’s only unsupported on CPUs with smaller E(fficeincy) cores. AVX-512 was never really supported in newer consumer CPUs with heterogeneous architecture. These CPUs have a mix of powerful cores and efficiency cores. The AVX-512 instructions were never added to the efficiency cores because it would use way too much die space and defeat the purpose of efficiency cores. T…

Catching the bad instruction fault on the E-cores and only scheduling the thread on the P-cores would be something that could be added to Linux (there were already third party patches towards that goal) if Intel had not disable the feature entirely.

Re: Removing characters from strings faster with AVX-512

#40

Post by Andoryuuta »

mhh__ wrote: post #24
Andoryuuta wrote: post #22

Intel is removing AVX-512 support from their newer CPU's (Alder Lake +). :/ https://www.igorslab.de/en/intel-deactivated-avx-512-on-alde...

You're forgetting about server CPUs, and we don't know yet about Raptor Lake.

Ah, yep. You're totally right. I didn't even consider server CPUs. Also, I thought I read somewhere that it was for all consumer CPUs starting at Alder Lake, but I have no idea where, so I could be entirely wrong. :)
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